Multiple Choice Answers

27. In skeletal muscle, glycogen phosphorylase is regulated both covalently and allosterically.
a. Write the reaction catalyzed by glycogen phosphorylase. (3)
b. Covalent modification of glycogen phosphorylase requires two different reactions.
       Write oneof the reactions. Name the enzyme that catalyzes the reaction and the
       conformation (R or T) that is stabilized as a result of the reaction. (4)
c. What is the effect on glycogen phosphorylase b activity of increasing [AMP]? (1)

      a. glycogen_(n+1) + P_i → glycogen_n + glucose-1-P

      b. PKA catalyzes this reaction:
      glycogen phosphorylase b–OH + ATP → glycogen phosphorylase a–P + ADP
      Glycogen phosphorylase b is stabilized in the T conformation, and glycogen
      phosphorylase a is stabilized in the R conformation.

      b. PP1 catalyzes this reaction:
      glycogen phosphorylase a–P + H₂O → glycogen phosphorylase b–OH + P_i

      c. AMP is a positive allosteric modulator of glycogen phosphorylase and increases its
     activity.

28. a. Write the complete oxidation-reduction reaction catalyzed by Complex II.
b. Calculate ΔE°′ for the reaction.
c. Calculate ΔG°′ for the reaction.
d. Since ΔG°′ for ATP hydrolysis = − 30.5 kJ/mole, how many ATP's can be synthesized,
     theoretically, as a result of the oxidation-reduction reaction catalyzed by Complex II?

      a. succinate + Q → fumarate + QH₂

      b. ΔE°′ = 0.045 V − 0.031 V = 0.014 V

      c. ΔG°′ = − (2) (96480 J/V-mole) (0.014 V) = − 2701.44 J/mol = − 2.7 kJ/mol

      d. none

29. The enzymes citrate synthase, aconitase, and isocitrate dehydrogenase catalyze the first
      three reactions in the citric acid cycle. (10)
a. Draw the molecules in the three reactions in order, starting with the substrate(s) for
      citrate synthase.
b. Choose two of the three enzymes and name the type of reaction each catalyzes.
     (oxidative decarboxylation, oxidation-reduction, substrate phosphorylation, etc.)

      b. Citrate synthase catalyzes a condensation reaction.
     Aconitase catalyzes an isomerization reaction.
     Isocitrate dehydrogenase catalyzes an oxidative decarboxylation reaction.

30. Pyruvate dehydrogenase (PDH) is a large enzyme complex in the mitochondrial matrix.
a. Write the overall reaction catalyzed by PDH. (4)
b. The diagram below shows an intermediate in the reaction. Which enzyme (E₁, E₂, or E₃)
      is involved, and what coenzyme has a bond to the intermediate? (2)
c. What happens next in the reaction? (2)

      a.                                                                                                                                                  

      c. CO₂ is released, and hydroxyethyl is transferred to lipoamide.

31. Each of the following questions is about regulation of carbohydrate metabolism.
Answer one (your choice). (6)
a. Write the reactions catalyzed by PFK-2 and FBPase-2.
     Explain why the reactions are coupled together.
b. Describe the effects of insulin and glucagon on the pathways of glycolysis and
      gluconeogenesis in a liver cell. Be specific about the enzymes affected.
c. The carriers GLUT2, T1, and T2 are found only in liver cells. What unique function
      does a liver cell have, and how are these proteins involved?

      a. PFK-2: fructose-6-P + ATP → fructose-2,6-bis-P + ADP
     FBPase-2: fructose-2,6-bis-P + H₂O → fructose-6-P + P_i
     These are coupled together because both enzymes are one protein, but only one
     enzyme is active at a time. Also because fructose-2,6-bis-P is an allosteric modulator of
     PFK-1 and FBPase-1, which means it helps regulate glycolysis and gluconeogenesis.

      b. Glucagon is secreted when blood glucose is low. It causes activation of PKA and,
      therefore, activation of FBPase-2 and inhibition of PFK-2. As a result (see answer to a)
     PFK-1 becomes less active, and glycolysis is less likely to occur. FBPase-1 is no longer
     inhibited by fructose-2,6-bis-P, so gluconeogenesis does occur. PKA also phosphorylates
     and inhibits pyruvate kinase, which also reduces glycolysis.

           Insulin is secreted when blood glucose is high. Insulin causes the activation of PP1
     which removes the phosphoryl groups transferred to enzymes by PKA, which reverses
     the effects of PKA. Glycolysis is more likely to occur, and gluconeogenesis is inhibited.

      c. Liver cells help in regulating blood glucose concentration. GLUT2 is a glucose carrier in
     the cell membrane which allows glucose to cross the membrane, moving down its
     concentration gradient. T1 and T2 are carriers in the endoplasmic reticulum (ER). T1
     transports glucose-6-P into the ER, where glucose-6-phosphatase removes the P. T2
     transports glucose out of the ER into the cytosol, where it can leave the cell via GLUT2.
      (Additional note: GLUT2 has a higher K_T than other GLUTs, allowing it to function at
     high [glucose].)

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